[BUG] Emojis in Markdown headers cause a crash

[bootloop-chris]

The bug

Including an emoji (specifically "🤖") in a markdown header causes a crash.

Minimal Repro

Tested against v5.0.1 and main@e3bae0071311d30a89b756919f1fe4800bb6a7cf

from textual.app import App, ComposeResult
from textual.widgets import Markdown


class TestApp(App):
    """Markdown append bug repro"""

    def compose(self) -> ComposeResult:
        yield Markdown("## 🤖 This heading is broken\n")

if __name__ == "__main__":
    app = TestApp()
    app.run()

Traceback

(captured in the console using markdown append, doesn't fail as loudly as the above repro but is easier to copy/paste the exception which is the same)

future: <_GatheringFuture finished exception=BadIdentifier("'heading-%F0%9F%A4%96-this-heading-is-broken-4381065296' is an invalid id; identifiers must contain only letters,
numbers, underscores, or hyphens, and must not begin with a number.")>
Traceback (most recent call last):
  File "/Users/chris/bootloop/textual/src/textual/widgets/_markdown.py", line 1416, in await_append
    new_blocks = list(self._parse_markdown(tokens))
  File "/Users/chris/bootloop/textual/src/textual/widgets/_markdown.py", line 1274, in _parse_markdown
    block.id = f"heading-{slug(block._content.plain)}-{id(block)}"
    ^^^^^^^^
  File "/Users/chris/bootloop/textual/src/textual/dom.py", line 783, in id
    check_identifiers("id", new_id)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/Users/chris/bootloop/textual/src/textual/dom.py", line 98, in check_identifiers
    raise BadIdentifier(
    ...<2 lines>...
    )
textual.dom.BadIdentifier: 'heading-%F0%9F%A4%96-this-heading-is-broken-4381065296' is an invalid id; identifiers must contain only letters, numbers, underscores, or hyphens, and
must not begin with a number.

Textual Diagnose Output

Textual Diagnostics

Versions

Name	Value
Textual	5.0.1
Rich	14.1.0

Python

Name	Value
Version	3.13.5
Implementation	CPython
Compiler	Clang 20.1.4
Executable	/Users/chris/bootloop/textual-bug-test/.venv/bin/python3

Operating System

Name	Value
System	Darwin
Release	24.5.0
Version	Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:43 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T8132

Terminal

Name	Value
Terminal Application	iTerm.app (3.5.14)
TERM	xterm-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=123, height=43
legacy_windows	False
min_width	1
max_width	123
is_terminal	True
encoding	utf-8
max_height	43
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

[github-actions]

We found the following entry in the FAQ which you may find helpful:

• Does Textual support images?

Feel free to close this issue if you found an answer in the FAQ. Otherwise, please give us a little time to review.

This project is developed and maintained by Will McGugan. Consider sponsoring Will's work on this project (and others).

This is an automated reply, generated by FAQtory

[TomJGooding]

Unfortunately not just emojis but other Unicode characters too. For example, a heading of Bibliothèque Textual will result in a similar crash.

[TomJGooding]

I'm surprised this hasn't been fixed yet in any of the Textual releases this week.

This bug was introduced in Textual v5.0.0 with the markdown streaming for LLMs. Aren't they a bit notorious for decorating heading with emojis?

[willmcgugan]

The slugs aren't actually required in a recent update to Markdown. It was more of a debug convenience. I can replace that slug/tracked slug implementation with a much simpler function now.

[3ls-it]

Hi Will. I notice that PR is still open though you seem to have squashed this bug. Thought I'd add my $.02.
I ran into this one in my own OpenAI Assistant client, which I recently re-wrote using Textual.
As pointed out above, LLMs regularly include Unicode characters in Markdown headers. (For me it was the use of curly double-quotes.)
Just upgraded to the latest Textual v5.3.0 and everything works great.

Thanks! I love Textual and Rich, great job.

[github-actions]

I hope I solved your issue!

Consider sponsoring my work to continue to maintain and develop Textual.

• Will McGugan